1. Field of the Invention
This invention relates to the extrusion of polytetrafluoroethylene (PTFE) tubing utilizing, in combination, a helically grooved extrusion tip and/or a helically grooved extrusion die. Such tubing, after removal of lubricant extrusion aid, can be expanded by stretching to produce an expanded, porous PTFE product having an angular orientation of the node and fibril microstructure. Products having increased strength compared to conventionally extruded and expanded tubes can be produced. Articles made by this process and apparatus are particularly suitable for use in the medical field, but they also have uses in nonmedical fields.
2. Description of the Prior Art
The products of this invention derive from paste-formed, extruded tubular products of PTFE. Paste extrusion or paste forming techniques are well known and consist of mixing a coagulated dispersion of PTFE resin with a liquid lubricant, preforming a tubular billet thereof, and forcing the mixture through an annular extrusion orifice in a ram-type batch extruder to form a coherent, extruded, unsintered PTFE tube. The lubricant is then removed, usually by a heat drying process, to form a porous, unsintered PTFE tube having a density usually within the range of 1.5 to 1.7 gm/cc. Such densities correspond to porosities of approximately 39% to 26%, respectively. The unsintered tube can be heated to a temperature above its crystalline melt point of about 345.degree. C. to sinter it thereby coalescing the porous material to form a nonporous, sintered PTFE tube.
U.S. Pat. No. 3,008,187, discloses a method of forming PTFE tubing having improved resistance to cracking and seepage of low viscosity fluids. That method consists of extruding a preformed, annular billet of a mixture of PTFE resin and a volatile extrusion aid through an extruder barrel provided with a centrally located mandrel, through a circular, tapered entry region and thence through an annular orifice to form a tube. Both a portion of the tapered circular inner surface of the tapered entry region and the tapered circular outer surface of the mandrel within the entry region are provided with a plurality of helical grooves. The grooves displace the fibers of tetrafluorethylene resin formed at the outer surface layer and the inner surface layer within the tapered entrance region.
U.S. Pat. No. 3,953,566 discloses and claims a process for making porous, expanded PTFE and U.S. Pat. No. 4,187,390 discloses and claims products of porous, expanded PTFE. The microstructure of those porous, uniaxially expanded PTFE products comprises nodes interconnected by fibrils in which the long axis of the nodes are oriented substantially perpendicularly to the direction of uniaxial expansion.
The products of U.S. Pat. No. 4,187,390 have high matrix tensile strengths, specifically, above 7,300 psi in at least one direction. Both U.S. Pat. No. 3,953,566 and U.S. Pat. No. 4,187,390 are incorporated herein by reference thereto.